The invention relates to a circuit device for operating a high-pressure discharge lamp with successive current phases, which circuit device is provided with means for operating the connected lamp by means of a dimming signal Ds from a first state to a second state, which circuit device is provided with:
input terminals for connecting the circuit device to a power supply source,
output terminals for connecting the lamp to be operated,
a switch-mode power supply (smps) with a converter provided with a semiconductor switch for generating a converter output voltage across buffer capacitor means having a capacitance, and
a control circuit for generating a control signal in dependence upon the dimming signal for controlling the switch.
A circuit device of the type mentioned in the opening paragraph is known from U.S. Pat. No. 5,828,178. In general, high-pressure discharge lamps are operated using successive current phases of periodically alternating polarity. In a frequently applied structure of the circuit device, the converter is connected, for this purpose, to a commutator, for example, in the form of a bridge circuit.
In the known circuit device, an optical sensor is provided for detecting light generated by the lamp. This enables a control for dimming the lamp to be realized.
The known dimming system for high-pressure discharge lamps, however, has a number of serious practical drawbacks. One serious drawback relates to the use of an optical sensor. On the one hand, because in order to properly detect the light generated by the lamp, correction for ambient light is required and, on the other hand, because such a detection is very sensitive to soiling of the sensor. An additional, frequently encountered problem relates to the fact that unstable burning of the lamp occurs easily during the dimming process, which may cause the lamp to start flickering or even cease burning altogether.
It is an object of the invention to provide a measure by means of which said drawbacks are counteracted.
To achieve this, a circuit device of the type mentioned in the opening paragraph is characterized, as a circuit device in accordance with the invention, in that the circuit device is provided with means for adapting the capacitance of the buffer capacitor. Surprisingly it has been found in experiments that by adapting the capacitance of the buffer capacitor means, at least during the transition from a first state to a second state of the lamp, both extinction and unstable burning of the lamp can be effectively counteracted. Particularly when operation of the lamp from the first state to the second state corresponds to a reduction of the power consumed by the lamp, it has been found that adaptation of the capacitance of the buffer capacitor means corresponding to a reduction of the capacitance is very effective in precluding unstable burning of the lamp or even extinction of the lamp.
Preferably, the capacitance of the buffer capacitor means is adapted in that the capacitance is built up of two or more electrically parallel-connected capacitors which can be individually switched off by means of a switch. The invention can particularly suitably be used in cases where the lamp is a metal halide lamp provided with a discharge vessel having a ceramic wall. The term xe2x80x9cceramic wallxe2x80x9d is to be taken to mean in the claims, a wall of a densely sintered metal oxide, such as aluminum oxide and YAG, and of densely sintered metal nitride, such as AlN, said wall being light-transmitting over at least a part of its surface.